The embodiments described herein generally relate to a CT system, and in particular to a calibration device and method and calibration method for CT system.
At present, a known CT system widely applied to the medical field can image individual parts of a patient so as to assist medical workers in diagnosis and treatment of diseases.
When the CT system is used, two critical steps should be carried out—aligning a tube with a collimator on a scan plane and aligning the tube, the collimator, and a detector on the scan plane. These two steps can also be called POR (Plane of Rotation) and BOW (BEAM on Window) alignments. The POR is used to put an x-ray beam parallel to the plane of rotation. This ensures that a focal spot of the tube is aligned with the center of the collimator by moving the tube in a Z-direction. BOW is used to put the x-ray beam at the center of the detector on a Z-axis after the POR by adjusting the detector in the Z-direction. The idea is to treat the focal spot of the tube and the collimator as a reference plane and enable the center of the detector to be coplanar with said reference plane.
FIG. 1 is a diagram of a single-slice CT system. As illustrated in FIG. 1, the CT system includes a tube 10, a collimator 11 and a detector 12. The plane of rotation refers to the plane on which the center of the tube 10, the center of the collimator 11, and the center of the detector 12 are aligned, and the z-axis direction refers to the direction perpendicular to the plane of rotation.
Currently, as for the POR, films are placed at the bottom of an aperture of a back cover of the CT system. The films are exposed twice respectively at 0 degree and 180 degrees of the x-ray tube. The difference of the x-ray profiles obtained by exposing the films twice is the amount of misalignment. The amount of misalignment is measured with a ruler or a micrometer. Then, according to the obtained amount of misalignment, the tube is adjusted so that the difference of the x-ray profiles obtained by two exposures is substantially 0, indicating that the POR has been aligned.
As for the BOW, three films are respectively placed on the left side, the center and the right side of channels of the detector. Then, these three films are exposed and checked to obtain the amount of misalignment in the three positions. Adjustment is then performed so that said amount of misalignment is substantially 0, i.e. it indicates that the BOW has been aligned.
The aforesaid technique is simple, effective and easy to be understood. However, the technique has following defects.
Firstly, because the aforesaid method needs to use films, but the films are one-off, generally speaking, at least 8 films are needed totally for the POR and BOW, wherein 4 films are used before alignment and another 4 films are used after alignment for double check. If the double check fails, more films may be needed. Therefore, the cost is high according to the prior technical method.
Secondly, because of use of the film, it is not good to environment.
Thirdly, because the amount of misalignment is measured with a ruler or a micrometer according to the prior technique, but the profile edge on the film is usually unclear in fact, the obtained result is not accurate.